Lash adjuster

ABSTRACT

A lash adjuster includes a nut member inserted in a receiving hole formed in the top surface of a cylinder head, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread on the inner periphery of the nut member, and a return spring biasing the adjusting screw. The adjusting screw has an end protruding from the nut member and pivotally supporting an arm of a valve gear. The adjusting screw is a solid member. A spring seat is disposed between the adjusting screw and the return spring and kept in point contact with the end of the adjusting screw inserted in the nut member. The spring seat is axially slidably fitted in the inner periphery of the nut member, thereby keeping the adjusting screw and the spring seat coaxial with each other.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a lash adjuster mounted in an engine valvegear.

2. Background Art

Known valve gears for moving a valve provided at an intake port or anexhaust port of an engine include one comprising an arm pivotable aboutone end thereof and adapted to be pushed down at its central portion,thereby pushing down a valve stem at the other end (swing arm type valvegear), one comprising an arm pivotable about its central portion andadapted to be pushed up by a cam at one end thereof, thereby pushingdown a valve stem at the other end (rocker arm type valve gear), and onecomprising a valve lifter vertically movably supported and adapted to bepushed down by a cam, thereby pushing down a valve stem (direct typevalve gear).

In these valve gears, gaps between their component parts may change dueto differences in thermal expansion between component parts, which maycause noise and compression leakage. Also, when the sliding parts of thevalve gear become worn too, gaps between component parts of the valvegear may change, which may also cause noise.

In order to prevent such noise and compression leakage, ordinary valveadjusters include a lash adjuster for absorbing gaps between componentparts of the valve gear.

One known lash adjuster used in a swing arm type valve gear comprises anut member inserted in a mounting hole formed in the top surface of acylinder head, an adjusting screw having an external thread on its outerperiphery which is in threaded engagement with an internal thread formedon the inner periphery of the nut member, and a return spring biasingthe adjusting screw in the direction to protrude upwardly from the nutmember, the adjusting screw pivotally supporting the arm of the valvegear with its end protruding from the nut member (JP Patent Publication2005-273510A).

One know lash adjuster used in a direct type valve gear comprises alifter body vertically slidably inserted in a guide hole formed in acylinder head, a nut member fixed to the lifter body so as to bevertically movable together with the lifter body, an adjusting screwhaving an external thread on its outer periphery which is in threadedengagement with an internal thread formed on the inner periphery of thenut member, and a return spring biasing the adjusting screw in thedirection to protrude downwardly from the nut member, the adjustingscrew pressing the valve stem of the valve gear with its end protrudingfrom the nut member (JP Patent Publication 2003-227318A).

One known lash adjuster used in a rocker arm type valve gear comprises anut member inserted in a mounting hole formed in the bottom surface ofthe arm which pivots as the cam rotates, an adjusting screw having anexternal thread on its outer periphery which is in threaded engagementwith an internal thread formed on the inner periphery of the nut member,and a return spring biasing the adjusting screw in the direction toprotrude downwardly from the nut member, the adjusting screw pressingthe valve stem of the valve gear with its end protruding from the nutmember (JP Patent Publication 2006-132426A).

In these lash adjusters, the external thread of the adjusting screw andthe internal thread of the nut member have a serration-shaped sectionsuch that their pressure flanks for receiving pressure when a load isapplied that tends to push the adjusting screw into the nut memberhaving a greater flank angle than their clearance flanks. Thus, as thecam rotates and the load is applied that tends to push the adjustingscrew into the nut member, the pressure flank of the external thread ofthe adjusting screw is supported on the pressure flank of the internalthread of the nut member, so that the adjusting screw is axially fixedin position.

If gaps between component parts of the valve gear change due e.g. tothermal expansion of the valve gear, the adjusting screw move axially inthe nut member while rotating, thus absorbing change in gaps betweencomponent parts of the valve gear.

SUMMARY OF THE INVENTION

With the lash adjuster disclosed in each of Patent documents 1 and 3,since the return spring is in direct contact with the end of theadjusting screw inserted in the nut member, there is the possibilitythat the adjusting screw may not be smoothly movable axially whilerotating due to the friction between the adjusting screw and the returnspring.

With the lash adjuster disclosed in Patent document 2, a spring seat isdisposed between the adjusting screw and the return spring to preventfriction between the adjusting screw and the return spring. This returnspring has a convex spherical surface kept in point contact with theadjusting screw.

The adjusting screw of this lash adjuster is a hollow member having anopening at its end inserted in the nut member. The spring seat is fittedin the adjusting screw so as to be rotatable relative to the adjustingscrew, thereby keeping the adjusting screw and the spring seat coaxialwith each other.

But with this lash adjuster, since the spring seat is fitted in theinner periphery of the adjusting screw, the outer periphery of thespring seat tends to contact the adjusting screw, which may in turn makesmooth rotation of the adjusting screw impossible due to frictionbetween the contact surfaces of the spring seat and the adjusting screw.

An object of the present invention is to provide a lash adjuster ofwhich the adjusting screw can rotate smoothly.

In order to achieve this object, a solid member is used as the adjustingscrew, a spring seat is disposed between the adjusting screw and thereturn spring so as to be kept in point contact with the end of theadjusting screw inserted in the nut member, and the spring seat isaxially slidably fitted in the inner periphery of the nut member,thereby keeping the adjusting screw and the spring seat coaxial witheach other.

If the spring seat is fitted in the internal thread on the innerperiphery of the nut member, the fitting surface of the spring seatfitted in the internal thread preferably has an axial length longer thanthe pitch of the internal thread.

The spring seat may comprise a cylindrical portion fitted in the innerperiphery of the nut member, and an end portion kept in point contactwith the end of the adjusting screw inserted in the nut member, with thereturn spring inserted in the cylindrical portion of the spring seat.

One of the contact surfaces of the adjusting screw and the spring seatmay be a convex spherical surface and the other of the contact surfacesmay be a flat surface; one of these contact surfaces may be a convexspherical surface and the other of the contact surfaces may be a concavespherical surface; or one of these contact surfaces may be a convexspherical surface and the other of the contact surfaces may also be aconvex spherical surface.

With the lash adjuster according to the present invention, since thespring seat is fitted in the inner periphery of the nut member, theouter periphery of the spring seat is kept out of contact with theadjusting screw, so that there is no friction between the outerperiphery of the spring seat and the adjusting screw. Also, even thoughthe outer periphery of the spring seat contacts the inner periphery ofthe nut member, this does not hinder rotation of the adjusting screw, sothat the adjusting screw can rotate smoothly.

Since the fitting surface of the spring seat fitted in the innerperiphery of the nut member has an axial length that is longer than thepitch of the internal thread on the inner periphery of the nut member,the fitting surface is guided by the internal thread over the entirecircumference thereof. Thus, the spring seat can be diametricallypositioned stably compared to a lash adjuster of which the fittingsurface has an axial length shorter than the pitch of the internalthread.

With the arrangement in which the spring seat comprises a cylindricalportion fitted in the inner periphery of the nut member, and an endportion kept in point contact with the end of the adjusting screwinserted in the nut member, with the return spring inserted in thecylindrical portion of the spring seat, since the fitting surface of thespring seat fitted in the inner periphery of the nut memberdiametrically overlaps with the return spring, it is possible to reducethe axial length of the lash adjuster.

With the arrangement in which one of the contact surfaces of theadjusting screw and the spring seat is a convex spherical surface andthe other is a flat surface, the contact surfaces can be formed easilyat a low cost.

With the arrangement in which one of the contact surfaces of theadjusting screw and the spring seat is a convex spherical surface andthe other is a concave spherical surface, the spring seat is less likelyincline, so that the contact position between the adjusting screw andthe spring seat stabilizes.

With the arrangement in which one of the contact surfaces of theadjusting screw and the spring seat is a convex spherical surface andthe other is also a convex spherical surface, it is possible to reducethe friction between the contact surfaces to an extremely low level,thus further reducing the resistance to rotation of the adjusting screw.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve gear including a lash adjusteraccording to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of a portion of the valve gearwhere there is the lash adjuster.

FIG. 3 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 2, i.e. the contact surface of spring seat is replaced by a concavespherical surface.

FIG. 4 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 2, i.e. the contact surface of the adjusting screw is replaced by aflat surface, and the contact surface of the spring seat is replaced bya convex spherical surface.

FIG. 5 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 2, i.e. the contact surface of the spring seat is replaced by aconvex spherical surface.

FIG. 6 is an enlarged sectional view of a modification in which thespring seat shown in FIG. 2 is replaced by a spring seat comprising acylindrical portion fitted in the inner periphery of the nut member, andan end portion kept in point contact with the end of the adjusting screwinserted in the nut member.

FIG. 7 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 6, i.e. the contact surface of spring seat is replaced by a concavespherical surface.

FIG. 8 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 6, i.e. the contact surface of the adjusting screw is replaced by aflat surface, and the contact surface of the spring seat is replaced bya convex spherical surface.

FIG. 9 is an enlarged sectional view of a modification in which one ofthe contact surfaces of the adjusting screw and the spring seat shown inFIG. 6, i.e. the contact surface of the spring seat is replaced by aconvex spherical surface.

FIG. 10 is a front view of a valve gear including a lash adjusteraccording to a second embodiment of the present invention.

FIG. 11 is a front view of a valve gear including a lash adjusteraccording to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a valve gear including the lash adjuster 1 according to thefirst embodiment of the present invention. This valve gear includes avalve 4 for an intake port 3 formed in a cylinder head 2 of an engine, avalve stem 5 connected to the valve 4, and an arm 7 that pivots as a cam6 rotates.

The valve stem 5 extends upwardly from the valve 4 and is slidablyinserted through the cylinder head 2. An annular spring retainer 8 isfixed to the outer periphery of the valve stem 5 at its upper portion. Avalve spring 9 is mounted between the bottom surface of the springretainer 8 and the top surface of the cylinder head 2. The valve spring9 biases the valve stem 5 upwardly through the spring retainer 8,thereby seating the valve 4 on a valve seat 10.

The arm 7 has one end thereof supported by the lash adjuster 1 and theother end in contact with the top end of the valve stem 5. The arm 7carries at its central portion a roller 11 kept in contact with the cam6, which is located over the arm 7.

As shown in FIG. 2, the lash adjuster 1 comprises a tubular nut member13 received in a receiving hole 12 formed in the top surface of thecylinder head 2, an adjusting screw 16 having an external thread 15 onits outer periphery at its lower portion which is in threaded engagementwith an internal thread 14 formed on the inner periphery of the nutmember 13, a return spring 17 biasing the adjusting screw 16, and aspring seat 18 disposed between the return spring 17 and the adjustingscrew 16.

The external thread 15 and the internal thread 14 have an asymmetricalserration-shaped section along the axis such that their pressure flanks19 for receiving pressure when a load is applied that tends to push theadjusting screw 16 into the nut member 13 having a greater flank anglethan their clearance flanks 20.

The return spring 17 has its bottom end supported on the bottom 21 ofthe nut member 13 and presses the adjusting screw 16 at its top end,thereby biasing the adjusting screw 16 in the direction to protrudeupwardly from the nut member 13.

The adjusting screw 16 is a solid member, and has a convex sphericalsurface 22 at its end inserted in the nut member 13. The spring seat 18has a flat surface 23 that is in contact with the convex sphericalsurface 22. The convex spherical surface 22 of the adjusting screw 16and the flat surface 23 of the spring seat 18 are in point contact witheach other at a point located on the rotation axis of the adjustingscrew 16.

The spring seat 18 is axially slidably fitted in the internal thread 14on the inner periphery of the nut member 13, and thus remains coaxialwith adjusting screw 16. The surface 24 of the spring seat 18 that isfitted in the internal thread 14 is a cylindrical surface of which theaxial length is longer than the pitch of the internal thread 14.

The spring seat 18 is formed with a protrusion 25 that is fitted in theinner periphery of the end portion of the return spring 17. By fittingthe protrusion 25 in the return spring 17, the return spring 17 and thespring seat 18 are kept coaxial with each other. If as shown, the returnspring 17 is a conical coil spring of which the diameter increasestoward the bottom 21 of the nut member 13, such a return spring 17 isless likely to buckle, which increases reliability of the return spring17.

The end 26 of the adjusting screw 16 protruding from the nut member 13is fitted in a recess 27 formed in the bottom surface of the arm 7 atthe one end, thus supporting the arm 7 so as to be pivotable about theprotruding end 26.

The operation of the lash adjuster 1 is now described.

When the cam 6 is rotated by the engine and the cam lobe 6 a of the cam6 presses down the arm 7, the valve 4 separates from the valve seat 10,thus opening the intake port 3. At this time, force acts on theadjusting screw 16 that tends to push in the adjusting screw 16. But dueto the frictional resistance between the external thread 15 of theadjusting screw 16 and the internal thread 14 of the nut member 13, theadjusting screw 16 is prevented from rotating, so that the adjustingscrew 16 is axially fixed in position.

When the cam 6 further rotates and the cam lobe 6 a moves past theroller 11, the valve stem 5 rises under the biasing force of the valvespring 9, until the valve 4 is seated on the valve seat 10 and theintake port 3 is closed.

In a strict sense, when the cam lobe 6 a of the cam 6 presses down thearm 7, slight slip occurs between the pressure flank 19 of the externalthread 15 and the pressure flank 19 of the internal thread 14. But afterthe cam lobe 6 a has moved past the roller 11 and until the cam lobe 6 aagain contacts the roller 11, since a load tending to push in theadjusting screw 16 is removed, the adjusting screw 16 returns to theoriginal position under the biasing force of the return spring 17.

When the distance between the cam 6 and the arm 7 increases due todifferences in thermal expansion between component parts of the valvegear such as the cylinder head 2, valve stem 5 and arm 7 while theengine is running, the adjusting screw 16 moves in the protrudingdirection while rotating under the biasing force of the return spring17. Thus, a gap never forms between the base circle 6 b of the cam 6 andthe roller 11.

Conversely, when the contact surfaces of the valve 4 and the valve seat10 become worn, even while the base circle 6 b of the cam 6 is incontact with the roller 11, the biasing force of the valve spring 9continuously acts on the adjusting screw 16 as a load tending to push inthe adjusting screw 16. Thus, due to slight slip that occurs between theexternal thread 15 and the internal thread 14 every time the cam lobe 6a contacts the roller 11, the adjusting screw 15 is gradually pushedinto the nut member, and the valve stem 5 gradually moves up, thuspreventing formation of a gap between the contact surfaces of the valve4 and the valve seat 10.

With this lash adjuster 1, since the spring seat 18 is fitted in theinner periphery of the nut member 13, the outer periphery of the springseat 18 is kept out of contact with the adjusting screw 16, so thatthere is no friction between the outer periphery of the spring seat 18and the adjusting screw 16. Also, even though the outer periphery of thespring seat 18 contacts the inner periphery of the nut member 13, thisdoes not hinder rotation of the adjusting screw 16, so that theadjusting screw 16 can rotate smoothly.

With this lash adjuster 1, since the fitting surface 24 of the springseat 18 has an axial length that is longer than the pitch of theinternal thread 14 on the inner periphery of the nut member 13, thefitting surface 24 is guided by the internal thread 14 over the entirecircumference thereof. Thus, the spring seat 18 can be diametricallypositioned stably compared to a lash adjuster of which the fittingsurface 24 has an axial length shorter than the pitch of the internalthread 14.

With this lash adjuster 1, since the adjusting screw 16 is a solidmember, its rigidity is high compared to the adjusting screw of a lashadjuster which is a hollow member having an opening at its end insertedin the nut member. Thus, if the external thread 15 on the outerperiphery of the adjusting screw 16 is formed by rolling, the externalthread 15 is less likely to be an incomplete thread.

In this embodiment, of the contact surfaces on the adjusting screw 16and the spring seat 18, the contact surface on the adjusting screw 16 isthe convex spherical surface 22 and the contact surface on the springseat 18 is the flat surface 23. But instead, as shown in FIG. 4, thecontact surface on the adjusting screw 16 may be a flat surface 28 andthe contact surface on the spring seat 18 may be a convex sphericalsurface 29. By forming a convex spherical surface on one of the contactsurfaces of the adjusting screw 16 and the spring seat 18 and forming aflat surface on the other, the contact surfaces can be easily formed ata low cost.

Alternatively, of the contact surfaces of the adjusting screw 16 and thespring seat 18, as shown in FIG. 3, the contact surface on the adjustingscrew 16 may be a convex spherical surface 30 and the contact surface onthe spring seat 18 may be a concave spherical surface 31 having a largerradius of curvature than the convex spherical surface 30. With thisarrangement, the spring seat 18 is less likely to incline, whichstabilizes the contact position between the adjusting screw 16 and thespring seat 18. Similarly, the contact surface on the spring seat 18 maybe a convex spherical surface (not shown) and the contact surface on theadjusting screw 16 may be a concave spherical surface (not shown).

Further alternatively, of the contact surfaces of the adjusting screw 16and the spring seat 18, as shown in FIG. 5, the contact surface on theadjusting screw 16 may be a convex spherical surface 32 and the contactsurface on the spring seat 18 may also be a convex spherical surface 33.With this arrangement, it is possible to reduce the friction between thecontact surfaces of the adjusting screw 16 and the spring seat 18 to anextremely low level, thus further reducing the resistance to rotation ofthe adjusting crew 16.

In the above embodiment, the solid spring seat 18 is fitted in the innerperiphery of the nut member 13 while being kept in point contact withthe end of the adjusting screw 16 inserted in the nut member 13. Butinstead of this spring seat 18, a spring seat 36 shown in FIG. 6 may beused, which comprises a cylindrical portion 34 fitted in the innerperiphery of the nut member 13, and an end portion 35 kept in pointcontact with the end of the adjusting screw 16 inserted in the nutmember 13, with the return spring 17 inserted in the cylindrical portion34 of the spring seat 36. With this arrangement, since the fittingsurface 37 of the spring seat 36 fitted in the inner periphery of thenut member 13 diametrically overlaps with the return spring 17, it ispossible to reduce the axial length of the lash adjuster 1.

Preferably, as shown in FIG. 6, the fitting surface 37 has an axiallength that is larger than twice the pitch of the internal thread 14With this arrangement, irrespective of the axial position of the springseat 36, the fitting surface 37 of the spring seat 36 is always guidedby the internal thread 14 at two or more axially spaced points. Thismakes the spring seat 36 less likely to incline, thus stabilizing thecontact position between the adjusting screw 16 and the spring seat 36.

When the spring seat 36 comprising the cylindrical portion 34 and theend portion 35 is used too, as shown in FIGS. 6 and 8, one of thecontact surfaces of the adjusting screw 16 and the spring seat 36 may bea convex spherical surface 38 and the other may be a flat surface 39 sothat the contact surfaces can be easily formed at a low cost.Alternatively, as shown in FIG. 7, one of the contact surfaces may be aconvex spherical surface 40 and the other may be a concave sphericalsurface 41. With this arrangement, by the contact between the convexspherical surface 40 and the concave spherical surface 41, it ispossible to prevent inclination of the spring seat 36. Further, as shownin FIG. 9, one of the contact surfaces may be a convex spherical surface42 and the other may also be a convex spherical surface 43. With thisarrangement, it is possible to reduce the friction between the contactsurfaces of the adjusting screw 16 and the spring seat 36 to anextremely low level, thus further reducing the resistance to rotation ofthe adjusting crew 16.

FIG. 10 shows a valve gear including the lash adjuster 51 according tothe second embodiment of the present invention. As with the firstembodiment, this valve gear includes a valve 54 provided at an intakeport 53 of a cylinder head 52, and a valve stem 55 connected to thevalve 54. The valve stem 55 extends upwardly from the valve 54. A springretainer 56 is fixed to its upper portion. The valve spring 57 biasesthe spring retainer 56 upwardly, thereby seating the valve 54 on a valveseat 58.

The lash adjuster 51 comprises a lifter body 60 vertically slidablyinserted in a guide hole 59 formed in the cylinder head 52, a nut member61 vertically movable together with the lifter body 60, an adjustingscrew 64 having an external thread 63 on the outer periphery thereofwhich is in threaded engagement with an internal thread 62 formed on theinner periphery of the nut member 61, a return spring 65 biasing theadjusting screw 64, and a spring seat 66 disposed between the adjustingscrew 64 and the return spring 65.

The lifter body 60 comprises a tubular portion 67 and an end wall 68closing the top end of the tubular portion 67. A hard shim 69 is fixedto the top surface of the end wall 68. A cam 70 is in contact with theshim 69. The nut member 61 is integrally formed at the central portionof the end wall 68, and has its top end closed by the shim 69.

The external thread 63 and the internal thread 62 each have a pressureflank for receiving pressure when a force is applied that tends to pushthe adjusting screw 64 into the nut member 61, the pressure flank havinga larger flank angle than the clearance flank.

The return spring 65 has its top end supported by the shim 69 andpresses at its bottom end the adjusting screw 64 through the spring seat66, thereby biasing the adjusting screw 64 in the direction to protrudedownwardly from the nut member 61. The end of the adjusting screw 64protruding from the nut member 61 presses the top end of the valve stem55.

The adjusting screw 64 is a solid member having a convex sphericalsurface at its end inserted in the nut member 61. The spring seat 66 hasa flat surface that contacts the convex spherical surface. The adjustingscrew 64 is in point contact with the spring seat 66 at the center ofrotation of the adjusting screw 64.

The spring seat 66 is axially slidably fitted in the inner periphery ofthe nut member 61, so that the adjusting screw 64 and the spring seat 66remain coaxial with each other. The surface of the spring seat 66 fittedin the internal thread 62 is a cylindrical surface having an axiallength longer than the pitch of the internal thread 62. The spring seat66 comprises a cylindrical portion 71 fitted in the inner periphery ofthe nut member 61, and an end portion 72 kept in point contact with theend of the adjusting screw 64 inserted in the nut member 61, with thereturn spring 65 inserted in the cylindrical portion 71.

With this lash adjuster 51, as in the first embodiment, since the springseat 66 is fitted in the inner periphery of the nut member 61, the outerperiphery of the spring seat 66 is kept out of contact with theadjusting screw 64, so that there is no friction between the outerperiphery of the spring seat 66 and the adjusting screw 64. Also, eventhough the outer periphery of the spring seat 66 contacts the innerperiphery of the nut member 61, this does not hinder rotation of theadjusting screw 64, so that the adjusting screw 64 can rotate smoothly.

With this lash adjuster 51, since the fitting surface of the spring seat66 has an axial length that is longer than the pitch of the internalthread 62 on the inner periphery of the nut member 61, the fittingsurface of the spring seat 66 is guided by the internal thread 62 overthe entire circumference thereof. Thus, the spring seat 66 can bediametrically positioned stably compared to a lash adjuster of which thefitting surface has an axial length shorter than the pitch of theinternal thread 62.

With this lash adjuster 51, since the adjusting screw 64 is a solidmember, its rigidity is high compared to the adjusting screw of a lashadjuster which is a hollow member having an opening at its end insertedin the nut member. Thus, if the external thread 63 on the outerperiphery of the adjusting screw 64 is formed by rolling, the externalthread 63 is less likely to be an incomplete thread.

In this embodiment, of the contact surfaces on the adjusting screw 64and the spring seat 66, the contact surface on the adjusting screw 64 isa convex spherical surface and the contact surface on the spring seat 66is a flat surface. But instead, as in the first embodiment, the contactsurface on the adjusting screw 64 may be a flat surface and the contactsurface on the spring seat 66 may be a convex spherical surface.Alternatively, one of the contact surfaces of the adjusting screw 64 andthe spring seat 66 may be a convex spherical surface and the other maybe a concave spherical surface, or one of these contact surfaces may bea convex spherical surface and the other may also be a convex sphericalsurface.

In this embodiment, in order to reduce the axial length of the lashadjuster 51, the spring seat 66 is used which comprises the cylindricalportion 71 fitted in the inner periphery of the nut member 61, and theend portion 72 kept in point contact with the end of the adjusting screw64 inserted in the nut member 61, with the return spring 65 inserted inthe cylindrical portion 71 of the spring seat 66. But instead, as in thefirst embodiment, a solid spring seat (not shown) may be fitted in theinner periphery of the nut member 61 and kept in point contact with theend of the adjusting screw 64 inserted in the nut member 61.

In this embodiment, the nut member 61 and the lifter body 60 areintegrally formed. But the nut member 61 may be a member separate fromthe lifter body 60 and fixed to the lifter body 60. What is important isthat the nut member is vertically movable together with the lifter bodywhen the lifter body moves vertically.

FIG. 11 shows a valve gear including the lash adjuster 81 according tothe third embodiment of the present invention. This valve gear includesa valve 84 provided at an intake port 83 of a cylinder head 82 of anengine, a valve stem 85 connected to the valve 84, and an arm 87pivotally supported about a pivot shaft 86. The valve stem 85 extendsupwardly from the valve 84, and has a spring retainer 88 fixed to itsupper portion. A valve spring 89 biases the spring retainer 88 upwardly,thereby seating the valve 84 on a valve seat 90.

The arm 87 has its central portion pivotally supported by the pivotshaft 86. The arm 87 carries at one end thereof a roller 92 kept incontact with a cam 91 so that the arm 87 pivots as the cam 91 rotates.The lash adjuster 81 is mounted to the other end of the arm 87.

The lash adjuster 81 comprises a nut member 93, an adjusting screw 94, areturn spring 95, and a spring seat 96 disposed between the adjustingscrew 94 and the return spring 95. The nut member 93 is inserted in ahole 97 extending vertically through the arm 87. The adjusting screw 94has an external thread 99 on its outer periphery that is in threadedengagement with an internal thread 98 formed on the inner periphery ofthe nut member 93.

The nut member 93 has a top end protruding from the top surface of thearm 87, and a tubular cap 100 having a closed top end is fitted on andfixed to the protruding top end portion of the nut member 93. The cap100 engages the top edge of the hole 97, thereby preventing the nutmember 93 from separating downwardly from the hole 97. The nut member 93has a flange 101 at its bottom end which is in abutment with the bottomsurface of the arm 87, and configured to receive upward force applied tothe nut member 93.

The external thread 99 and the internal thread 98 each have a pressureflank for receiving pressure when a force is applied that tends to pushthe adjusting screw 94 into the nut member 93, the pressure flank havinga larger flank angle than the clearance flank.

The return spring 95 has its top end supported by the cap 100 andpresses at its bottom end the adjusting screw 94, thereby biasing theadjusting screw 94 in the direction to protrude downwardly from the nutmember 93. The end of the adjusting screw 94 protruding from the nutmember 93 is pressed against the top end of the valve stem 85.

The adjusting screw 94 is a solid member having a convex sphericalsurface at its end inserted in the nut member 93. The spring seat 96 hasa flat surface that contacts the convex spherical surface. The adjustingscrew 94 is in point contact with the spring seat 96 at the center ofrotation of the adjusting screw 94.

The spring seat 96 is axially slidably fitted in the inner periphery ofthe nut member 93, so that the adjusting screw 94 and the spring seat 96remain coaxial with each other. The spring seat 96 comprises acylindrical portion 102 fitted in the inner periphery of the nut member93, and an end portion 103 kept in point contact with the end of theadjusting screw 94 inserted in the nut member 93, with the return spring95 inserted in the cylindrical portion 102.

With this lash adjuster 81, as in the first embodiment, since the springseat 96 is fitted in the inner periphery of the nut member 93, the outerperiphery of the spring seat 96 is kept out of contact with theadjusting screw 94, so that there is no friction between the outerperiphery of the spring seat 96 and the adjusting screw 94. Also, eventhough the outer periphery of the spring seat 96 contacts the innerperiphery of the nut member 93, this does not hinder rotation of theadjusting screw 94, so that the adjusting screw 94 can rotate smoothly.

With this lash adjuster 81, since the adjusting screw 94 is a solidmember, its rigidity is high compared to the adjusting screw of a lashadjuster which is a hollow member having an opening at its end insertedin the nut member 93. Thus, if the external thread 99 on the outerperiphery of the adjusting screw 94 is formed by rolling, the externalthread 99 is less likely to be an incomplete thread.

In this embodiment, of the contact surfaces on the adjusting screw 94and the spring seat 96, the contact surface on the adjusting screw 94 isa convex spherical surface and the contact surface on the spring seat 96is a flat surface. But instead, as in the first embodiment, the contactsurface on the adjusting screw 94 may be a flat surface and the contactsurface on the spring seat 96 may be a convex spherical surface.Alternatively, one of the contact surfaces of the adjusting screw 94 andthe spring seat 96 may be a convex spherical surface and the other maybe a concave spherical surface, or one of these contact surfaces may bea convex spherical surface and the other may also be a convex sphericalsurface.

In this embodiment, in order to reduce the axial length of the lashadjuster 81, the spring seat 96 is used which comprises the cylindricalportion 102 fitted in the inner periphery of the nut member 93, and theend portion 103 kept in point contact with the end of the adjustingscrew 94 inserted in the nut member 93, with the return spring 95inserted in the cylindrical portion 102 of the spring seat 96. Butinstead, as in the first embodiment, a solid spring seat may be fittedin the inner periphery of the nut member 93 and kept in point contactwith the end of the adjusting screw 94 inserted in the nut member 93.

In any of the above embodiments, any of the adjusting screws 16, 64 and94, as well as any of the nut members 13, 61 and 93 and any of thespring seats 18, 36, 66 and 96 may be made of a ferrous material, withtheir surfaces subjected to carburization, carbonitriding or softnitriding to increase their durability.

1. A lash adjuster comprising a nut member inserted in a receiving holeformed in a top surface of a cylinder head and having an internal threadon its inner periphery, an adjusting screw having an external thread onits outer periphery which is in threaded engagement with the internalthread of the nut member and a return spring biasing the adjusting screwin a direction to protrude upwardly from the nut member, said adjustingscrew having an end protruding from the nut member and pivotallysupporting an arm of a valve gear, wherein said adjusting screw is asolid member, that the lash adjuster further comprises a spring seatdisposed between the adjusting screw and the return spring and kept inpoint contact with an end of the adjusting screw inserted in the nutmember, and the spring seat is axially slidably fitted in the innerperiphery of the nut member, thereby keeping the adjusting screw and thespring seat coaxial with each other.
 2. A lash adjuster comprising alifter body vertically slidably inserted in a guide hole formed in acylinder head, a nut member vertically movable together with the lifterbody and having an internal thread on its inner periphery, an adjustingscrew having an external thread on its outer periphery which is inthreaded engagement with the internal thread of the nut member, and areturn spring biasing the adjusting screw in a direction to protrudedownwardly from the nut member, said adjusting screw having an endprotruding from the nut member and pressing a valve stem of a valvegear, wherein said adjusting screw is a solid member, the lash adjusterfurther comprises a spring seat disposed between the adjusting screw andthe return spring and kept in point contact with an end of the adjustingscrew inserted in the nut member, and that the spring seat is axiallyslidably fitted in the inner periphery of the nut member, therebykeeping the adjusting screw and the spring seat coaxial with each other.3. A lash adjuster comprising a nut member inserted in a receiving holeformed in a bottom surface of an arm that pivots as a cam rotates andhaving an internal thread on its inner periphery, an adjusting screwhaving an external thread on its outer periphery which is in threadedengagement with the internal thread of the nut member, and a returnspring biasing the adjusting screw in a direction to protrude downwardlyfrom the nut member, said adjusting screw having an end protruding fromthe nut member and pressing a valve stem of a valve gear, wherein saidadjusting screw is a solid member, the lash adjuster further comprises aspring seat disposed between the adjusting screw and the return springand kept in point contact with an end of the adjusting screw inserted inthe nut member, and that the spring seat is axially slidably fitted inthe inner periphery of the nut member, thereby keeping the adjustingscrew and the spring seat coaxial with each other.
 4. The lash adjusterof claim 1 wherein the spring seat has a fitting surface axiallyslidably fitted in the internal thread on the inner periphery of the nutmember, said fitting surface having an axial length longer than a pitchof the internal thread.
 5. The lash adjuster of claim 1 wherein thespring seat comprises a cylindrical portion fitted in the innerperiphery of the nut member, and an end portion kept in point contactwith the end of the adjusting screw inserted in the nut member, andwherein the return spring is inserted in the cylindrical portion of thespring seat.
 6. The lash adjuster of claim 1 wherein one of contactsurfaces of the adjusting screw and the spring seat is a convexspherical surface and the other of the contact surfaces is a flatsurface.
 7. The lash adjuster of claim 1 wherein one of contact surfacesof the adjusting screw and the spring seat is a convex spherical surfaceand the other of the contact surfaces is a concave spherical surface. 8.The lash adjuster of claim 1 wherein one of contact surfaces of theadjusting screw and the spring seat is a convex spherical surface andthe other of the contact surfaces is also a convex spherical surface. 9.The lash adjuster of claim 2 wherein the spring seat has a fittingsurface axially slidably fitted in the internal thread on the innerperiphery of the nut member, said fitting surface having an axial lengthlonger than a pitch of the internal thread.
 10. The lash adjuster ofclaim 3 wherein the spring seat has a fitting surface axially slidablyfitted in the internal thread on the inner periphery of the nut member,said fitting surface having an axial length longer than a pitch of theinternal thread.
 11. The lash adjuster of claim 2 wherein the springseat comprises a cylindrical portion fitted in the inner periphery ofthe nut member, and an end portion kept in point contact with the end ofthe adjusting screw inserted in the nut member, and wherein the returnspring is inserted in the cylindrical portion of the spring seat. 12.The lash adjuster of claim 3 wherein the spring seat comprises acylindrical portion fitted in the inner periphery of the nut member, andan end portion kept in point contact with the end of the adjusting screwinserted in the nut member, and wherein the return spring is inserted inthe cylindrical portion of the spring seat.
 13. The lash adjuster ofclaim 2 wherein one of contact surfaces of the adjusting screw and thespring seat is a convex spherical surface and the other of the contactsurfaces is a flat surface.
 14. The lash adjuster of claim 3 wherein oneof contact surfaces of the adjusting screw and the spring seat is aconvex spherical surface and the other of the contact surfaces is a flatsurface.
 15. The lash adjuster of claim 2 wherein one of contactsurfaces of the adjusting screw and the spring seat is a convexspherical surface and the other of the contact surfaces is a concavespherical surface.
 16. The lash adjuster of claim 4 wherein one ofcontact surfaces of the adjusting screw and the spring seat is a convexspherical surface and the other of the contact surfaces is a concavespherical surface.
 17. The lash adjuster of claim 2 wherein one ofcontact surfaces of the adjusting screw and the spring seat is a convexspherical surface and the other of the contact surfaces is also a convexspherical surface.
 18. The lash adjuster of claim 3 wherein one ofcontact surfaces of the adjusting screw and the spring seat is a convexspherical surface and the other of the contact surfaces is also a convexspherical surface.